1. Field of the Invention
The present invention relates to a method for realizing a primary photometric standard of optical radiation, and more particularly, to a method for realizing a primary photometric standard of a photometric unit, such as the candela (cd) or lumen (lm) using a silicon photodiode.
2. Description of Related Art
Conventionally, the primary standard of the photometric units is realized by using a black body furnace made based on the Planck's law of radiation, an absolute radiometer for measuring an optical power by substituting the optical power with an electric power, or a synchrotron. In these methods, in order to measure an optical power at an accuracy which is high enough for a primary standard, it is necessary to provide large-scale equipment and employ high technology techniques, and it can be performed only in the national research organizations. However, it was apparent from the result of recent international comparison that the accuracy obtained by the primary standard provided by the national research organizations is only about .+-.1 % (See a reference 1 : Metrologia, Vol. 24, 1987, pp157). Therefore, it has been desired to improve the accuracy of the primary standard provided thereby.
Further, the primary standard realized as described above becomes the national standard, and is provided to the industrial level through a public organization. In the industrial level, the standard management section of each company etc. performs the calibration of measuring instruments. In such a traceability process, since it is repeated to transfer a luminous intensity of standard lamps referring to the higher level standard lamps, the accuracy obtained in the end measuring instrument becomes lower.
In order to overcome the aforementioned problems, there has been developed a new method which is called "the self-calibration method using a silicon photodiode" as a method for realizing the primary standard of the optical radiation with a high accuracy (See a reference 2 : Applied Optics Vol. 19, No. 8, 1980, pp1214). In the self-calibration method using the silicon photodiode, an absolute responsivity [A/W]of the silicon photodiode is obtained by measuring the surface reflectance and the internal quantum efficiency of the silicon photodiode, and it is confirmed that an accuracy in the order of 0.1 % is obtained, which is one digit higher than that obtained by the conventional primary standard of the optical radiation. This method requires comparatively simple equipment. It has been supposed that the aforementioned self-calibration method using the silicon photodiode is promising as a new method for realizing the primary photometric standard.
However, since the conventional self-calibration method is fundamentally the method for obtaining the absolute responsivity [A/W] at one wavelength of a monochromatic light by projecting the monochromatic light onto the silicon photodiode, in order to realize the photometric standard, it is necessary to measure the absolute responsivity R(.lambda.) over the entire visible spectrum using the self-calibration method. After measuring the absolute responsivity R(.lambda.), there can be calculated the responsivity R.sub.LF [A/lm] for the photometric quantity using the following equation by combination of the silicon photodiode and a filter for spectral luminous efficacy correction whose spectral transmittance .tau.(.lambda.) has been accurately measured, resulting in a photometric standard. ##EQU1## where
P(.lambda.) is a relative spectral distribution of a light source,
V(.lambda.) is a standard spectral luminous efficiency, and
K.sub.m is a maximum spectral luminous efficacy (683 [lm/W]).
However, in this method, in order to obtain the absolute spectral responsivity on the visible spectrum, it is necessary to prepare many light sources for projecting monochromatic lights of various wavelengths, and to perform the measurement according to the self-calibration method at the respective wavelengths. Accordingly, it is necessary to prepare laser light sources having many wavelengths and the equipment of the self-calibration method. Further, since it takes a long time to perform the aforementioned measurement, the conventional self-calibration method is not a simple measuring method for practical use.